1. Field of the Invention
The invention relates, in general, to the area of art known as null steering and, more particularly, to a method and apparatus for receiving radio signals from a desired source while at the same time reducing the response to interfering signals, if there is a difference in the angle of arrival of the two signals.
2. Description of Prior Art
Highly directive transmission and receiving beams have been developed and are used extensively in the antenna art. One type of directive beam antenna is the well-known "phased array", wherein array elements phase commands are used to scan the beam of a planar phased array. This technique is typified by U.S. Pat. Nos. 3,877,012 to Nelson, 3,806,930 to Gobert, and 3,319,249 to Blachier. Although "phased arrays" use phase adjustments, they do not relate to null steering art contemplated by the present invention.
In addition there have been developments in the area of both simultaneous formation of a null in the pattern of reception and for changing the direction of a null. This technique is usually termed "null steering phased arrays" or "adaptive arrays." Prior patents in this area include U.S. Pat. Nos. 3,670,335 to Hirsch; 3,964,065 to Roberts; and 3,725,929 to Spanos. The creation of pattern nulls in phased antenna arrays require devices for varying phase and amplitude of the signals received from or fed to each antenna of the array. The correct values of signal amplitude and phase relationship to be fed to each antenna are then calculated. By means of attenuators to adjust amplitude and phase shifters to adjust phase, the correct values are obtained. Thus, to create a null in a desired direction, a calculation and an adjustment to the attenuator and phase shifter for each antenna is required.
It is also known to use phase shifters alone and still obtain a pattern null in the desired direction. The adjustments, however, were very complicated. Since no exact solution was available from theory, the phase shifters for each antenna were set at the value obtained from the previous theory of amplitude and phase adjustment. The attenuator device was not needed since the amplitude was not varied. The phase shifter of one antenna was then changed a small amount. If the amplitude of the interfering signal decreased, then the change was deemed proper. If the interference signal increased, the phase shifter was changed in the opposite direction. This trial and error method was then repeated for each antenna of the array. The whole procedure was repeated over and over again until a stable result was obtained. This process, an iterative procedure, requires many rapid calculations which are time consuming unless a relatively high speed computer is available. A high speed accurate phase shifter for each antenna is required along with a highly sophisticated control system. In addition, since decisions have to be made very rapidly for each phase change, the criteria used to evaluate the null depth are very limited.